Hydrothermal synthesis of the perovskite manganites Pr{sub 0.5}Sr{sub 0.5}MnOâ and Nd{sub 0.5}Sr{sub 0.5}MnOâ and alkali-earth manganese oxides CaMnâOâ, 4H-SrMnOâ, and 2H-BaMnOâ